Piezo-electric transducer unit

ABSTRACT

An insulating substrate (1) is a laminate of a first insulating substrate (1a) and a second insulating substrate (1b) on which wires and electrodes are formed previously. In this case a concavity is formed on the insulating substrate (1) by a hollow part (4) of the first insulating substrate (1a) and an upper surface of the second insulating substrate. Metallic wires are also arranged on an area corresponding to a bottom surface of the concavity of the second insulating substrate. As a result, space for arranging metallic wires expands and a space between metallic wires can be widened. Further, metallic wires which bypass areas corresponding to external electrodes formed on a lower surface of the second insulating substrate can be formed. This results in smaller stray capacitance between an electrode and a wire and between wires.

TECHNICAL FIELD

This invention relates to a surface mounted piezo-electric transducerunit and, more particularly, to an arrangement of the internal wiringthereof.

BACKGROUND ART

In recent years, surface mounted piezo-electric transducer units whichcan be easily mounted on a surface of a printed-circuit board are usedfor piezo-electric transducers such as crystal oscillators for clockingsources of personal computers, communication devices, and the like. Inthese surface mounted piezo-electric transducers, the followingattempted improvements are publicly known:

(1) improving the oscillation characteristic of a piezo-electrictransducer corresponding to a change in temperature by changing doubleand support into cantilever-like support or reducing stress caused bythe difference in thermal expansion between the transducer and itscontainer.

(2) improving piezo-electric transducer production efficiency by formingon its longer sides two pairs of electrodes for connecting it to acontainer and thus enabling the transducer and container to be assembledregardless of the directions of their longer sides when the transduceris automatically fitted into the container.

A conventional piezo-electric transducer unit will be described belowwith reference to the drawings.

FIG. 1 is a perspective view of components of a conventionalpiezo-electric transducer unit. A piezo-electric transducer unit 100comprises a substrate 101 of ceramic insulating materials, athickness-mode crystal oscillator 102 (hereinafter referred to as "ATcrystal oscillator 102"), and a ceramic cap 103. On an upper surface ofthe substrate 101, there are a rectangular concavity 104 for housing theAT crystal oscillator 102 and separating grooves 105 and 106 forseparating conductive adhesives. Each of the separating grooves 105 and106 is on opposite sides of a centerline of the substrate 101 parallelto its shorter sides. The AT crystal oscillator 102 is of the so-calledconvex type and has an excitation electrode 121 and outgoing electrodes122 and 123 connected thereto, which are mounted fast on its upper andlower surfaces by vacuum evaporation, sputtering, and so forth. The ATcrystal oscillator 102 may be a rectangular AT plate of uniformthickness. The AT crystal oscillator 102 is housed in the concavity 104on the substrate 101, and the outgoing electrodes 122 and 123 are fixed,with conductive adhesives, onto connecting electrodes 107 and 110 or 108and 109 respectively. Then frequency adjustment is performed asnecessary and the cap 103 is joined onto the substrate 101, whichcompletes the crystal oscillator unit 100.

FIGS. 2, 3, and 4 are diagrams showing the state of metallic wire on thesubstrate 101. FIG. 2 is a front view, FIG. 3 is a view from thedirection of an arrow 130 in FIG. 2, and FIG. 4 is a rear view.

A surface (hereinafter referred to as "the mounting surface") of thesubstrate 101 shown in FIG. 2 is where the AT crystal oscillator 102 ismounted, and is nearly square. For convenience of the followingexplanation, it is assumed that a centerline of the near square parallelto its longer sides is an X-axis and that a centerline of the nearsquare parallel to its shorter sides is a Y-axis. The concavity 104 isalso nearly square and its center lines are identical to the above X-and Y-axis respectively. There are connecting electrodes at positionscontiguous to four corners of the concavity 104. An X-Y plane formed bythe above X- and Y-axis has four quadrants each of which has oneconnecting electrode. That is, the first quadrant has a first connectingelectrode 107; the second quadrant has a second connecting electrode108; the third quadrant has a third connecting electrode 109; and thefourth quadrant has a fourth connecting electrode 110.

The separating groove 105 lies both on the first quadrant and on thefourth quadrant, and is arranged between the first connecting electrode107 and fourth connecting electrode 110 for preventing these electrodesfrom short-circuiting. The separating groove 106 lies both on the secondquadrant and on the third quadrant, and is arranged between the secondconnecting electrode 108 and third connecting electrode 109 forpreventing these electrodes from short-circuiting.

On the reverse of the substrate 101, as shown in FIG. 4, there are afirst external electrode 111, a second external electrode 112, a thirdexternal electrode 113, and a fourth external electrode 114, thelocations of which correspond to the first, second, third, and fourthquadrant on the upper surface of the above substrate 101 respectively.

The first connecting electrode 107 and second connecting electrode 108are connected by a first metallic wire 115 and the first metallic wire115 is connected to the second external electrode 112 by a secondmetallic wire 116. The second metallic wire 116 reaches from themounting surface of the substrate 101 to the second external electrode112 on the reverse through a side of the substrate 101 corresponding tothe longer side of the mounting surface. The second connecting electrode108 is connected to the third external electrode 113 by a third metallicwire 117. The third metallic wire 117 extends from the second quadranthaving the second connecting electrode 108 to the third quadrant andreaches the third external electrode 113 through a side of the substrate101 by bypassing the third connecting electrode 109. As described above,the first and second connecting electrodes 107 and 108 and the secondand third external electrodes 112 and 113 are connected by the first,second, and third metallic wires 115, 116, and 117, which form a firstgroup of the wiring pattern.

The third connecting electrode 109 and fourth connecting electrode 110are connected by a fourth metallic wire 118 and the fourth metallic wire118 is connected to the fourth external electrode 114 by a fifthmetallic wire 119. The fifth metallic wire 119 reaches from the mountingsurface of the substrate 101 to the fourth external electrode 114 on thereverse through a side of the substrate 101 corresponding to the longerside of the mounting surface. The fourth connecting electrode 110 isconnected to the first external electrode 111 by a sixth metallic wire120. The sixth metallic wire 120 extends from the fourth quadrant havingthe fourth connecting electrode 110 to the first quadrant and reachesthe first external electrode 111 through a side of the substrate 101 bybypassing the first connecting electrode 107. As described above, thethird and fourth connecting electrodes 109 and 110 and the fourth andfirst external electrodes 114 and 111 are connected by the fourth,fifth, and sixth metallic wires 118, 119, and 120, which form a secondgroup of the wiring pattern.

In this conventional piezo-electric transducer unit, metallic wire isarranged close on the upper surface of the substrate 101. As a result asegment m₀ having narrow space d₀ between metallic wires belonging tothe different groups becomes larger, as shown in FIG. 2. Metallic wiresbelonging to the different groups differ in electric polarity, and solarge segment m₀ having narrow space d₀ between metallic wires willgenerate stray capacitance there. Furthermore, if the externalelectrodes and metallic wires arranged on the obverse and reverse of thesubstrate 101 belong to the different groups, stray capacitance willgenerate there. This stray capacitance increases equivalent parallelcapacitance of the above piezo-electric transducer unit.

The equivalent parallel capacitance will be described according toequivalent circuit diagrams of a piezo-electric transducer unit of FIGS.5 and 6. The equivalent circuit shown in FIG. 5 comprises a parallelcircuit of a series circuit having an inductance L, resonance resistancevalue R, and capacitance value C₁, capacitance value Cox of thepiezo-electric transducer, and the above stray capacitance value Copbetween wires. In the equivalent circuit of FIG. 6, the capacitance Coxof the piezo-electric transducer and stray capacitance Cop between wiresconnected in parallel in the equivalent circuit of FIG. 5 are combinedinto one parallel capacitance value Co. The parallel capacitance valueCo is given by:

    Co=Cox+Cop                                                 (1)

As described above, a stray capacitance value Cop of the conventionalpiezo-electric transducer unit is great. Therefore, there was theproblem of generating great parallel capacitance Co. When thispiezo-electric transducer unit is used in an oscillation circuit, greatparallel capacitance may produce the problem of delay in an oscillationstart or unstable oscillation.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a piezo-electrictransducer unit having improved characteristics by reducing straycapacitance.

In order to achieve the above object, a piezo-electric transducer unitaccording to the present invention is intended that external electrodesand wires having different polarities are not arranged on the obverseand reverse of a substrate. Specifically, there is a first excitingelectrode on a first surface of a piezo-electric transducer and a secondexciting electrode on a second surface, the back of the first surface.Furthermore, the piezo-electric transducer unit has at least one firstconnecting electrode connected to the first exciting electrode in thecase of the piezo-electric transducer being mounted, at least one secondconnecting electrode connected to the second exciting electrode in thecase of the piezo-electric transducer being mounted, at least one firstexternal electrode and at least one second external electrode on theback of the mounting surface of the piezo-electric transducer, a firstwire for connecting the first connecting electrode and the firstexternal electrode, and a second wire for connecting the secondconnecting electrode and the second external electrode. Thepiezo-electric transducer unit has a cap for covering the piezo-electrictransducer mounting surface of the substrate and sealing, with thesubstrate, the piezo-electric transducer with the piezo-electrictransducer mounted on the substrate. Also, the first and second externalelectrodes and the first and second wires are arranged so that, whenthey are projected onto a plane parallel to the piezo-electrictransducer mounting surface of the substrate, the shadows of the firstexternal electrode and the second wire do not overlap and the shadows ofthe second external electrode and the first wire do not overlap.

This arrangement prevents external electrodes and wires having differentpolarities from being arranged on the obverse and reverse of asubstrate, resulting in smaller stray capacitance.

Also, the substrate may have a concavity in a nearly central position ofthe piezo-electric transducer mounting surface for housing at least partof the piezo-electric transducer.

Also, in order to achieve the above configuration, it is good to performwiring on the bottom surface of the concavity on the substrate. Toachieve this, it is desirable that the substrate be divided into twosections and that wiring and forming electrodes be individuallyperformed. According to a preferred aspect, the substrate is dividedinto a first substrate and second substrate by a plane parallel to thepiezo-electric transducer mounting surface. The first substrate has aperforated aperture corresponding to the concavity, the first and secondconnecting electrodes, and holes perforating through the first substrateat the first and second connecting electrodes. The second substrate hasthe first and second external electrodes, the first and second wires,and perforating hole joints formed at positions corresponding to theperforating holes of the first substrate. Portions of the first andsecond wires are arranged at a position corresponding to the bottomsurface of the concavity so that, when the first and second externalelectrodes and the first and second wires are projected onto a planeparallel to the piezo-electric transducer mounting surface of thesubstrate, the shadows of the first external electrode and the secondwire do not overlap and the shadows of the second external electrode andthe first wire do not overlap. In this way the first and second wiresavoid the external electrodes.

Further, in a preferred embodiment of the present invention, theexternal shape of the substrate and the concavity are nearly square. Twofirst and two second connecting electrodes are provided arranged on thefour corners of the nearly square concavity so that they are symmetricalwith respect to a centerline parallel to the shorter sides of the nearsquare. Two first and two second external electrodes are arranged on thetwo longer sides of the nearly square substrate with one first andsecond external electrode per side so that they are symmetrical withrespect to a centerline parallel to the longer sides of the near square.

Further, in another embodiment of the present invention, the total areaof the first wire, the first external electrode, and the firstconnecting electrode is equal to that of the second wire, the secondexternal electrode, and the second connecting electrode.

Further, in still another embodiment of the present invention, the areaof the first wire is equal to that of the second wire.

Further, in another piezo-electric transducer unit according to thepresent invention, a space between wires with different polarities iswide and only a small segment has a comparatively narrow space betweenwires with different polarities. To achieve this, wires are alsoarranged on a bottom surface of a concavity on a substrate.Specifically, there is a first exciting electrode on a first surface ofa piezo-electric transducer and a second exciting electrode on a secondsurface, the back of the first surface. Furthermore, the substrate onwhich the piezo-electric transducer is mounted has a concavity in anearly central position of a piezo-electric transducer mounting surfacefor housing at least part of the piezo-electric transducer. Also, thepiezo-electric transducer unit has at least one first connectingelectrode connected to the first exciting electrode in the case of thepiezo-electric transducer being mounted, at least one second connectingelectrode connected to the second exciting electrode in the case of thepiezo-electric transducer being mounted, at least one first externalelectrode and at least one second external electrode on the back of themounting surface of the piezo-electric transducer, a first wire forconnecting the first connecting electrode and the first externalelectrode, and a second wire for connecting the second connectingelectrode and the second external electrode. The piezo-electrictransducer unit also has a cap for covering the piezo-electrictransducer mounting surface of the substrate and sealing, together withthe substrate, the piezo-electric transducer with the piezo-electrictransducer mounted on the substrate. Also, the substrate is divided intoa first substrate and second substrate by a plane parallel to thepiezo-electric transducer mounting surface. The first substrate has aperforated aperture corresponding to the concavity, the first and secondconnecting electrodes, and holes perforating through the first substrateat the first and second connecting electrodes. The second substrate hasthe first and second external electrodes, the first and second wires,and perforating hole joints formed at positions corresponding to theperforating holes of the first substrate. At least part a portion of thefirst and second wires are arranged on the second substrate surface atpositions corresponding to the bottom surface of the concavity.

In this configuration wires are also arranged on the bottom surface ofthe concavity on the substrate, which creates a large space for a wiringpattern and enabling space between wires of opposite polarity to bewidened.

Further, in a preferred embodiment of the present invention, theexternal shape of the substrate and the concavity are nearly square. Twofirst and two second connecting electrodes are arranged on the fourcorners of the nearly square concavity so that they are symmetrical withrespect to a centerline parallel to the shorter sides of the nearsquare. Two first and two second external electrodes are arranged on thetwo longer sides of the nearly square substrate with one first andsecond external electrode per side so that they are symmetrical withrespect to a centerline parallel to the longer sides of the near square.

Further, in another preferred embodiment of the present invention, thefirst and second wires do not cross a centerline parallel to the longersides of the concavity inside the concavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of components of a conventionalpiezo-electric transducer unit.

FIG. 2 is a view showing wiring and the arrangement of electrodes of aconventional piezo-electric transducer unit.

FIG. 3 is a view showing wiring and the arrangement of electrodes of aconventional piezo-electric transducer unit.

FIG. 4 is a view showing the arrangement of electrodes of a conventionalpiezo-electric transducer unit.

FIG. 5 is an equivalent circuit of a piezo-electric transducer unit.

FIG. 6 is an equivalent circuit of a piezo-electric transducer unit.

FIG. 7 is a perspective view of components of a piezo-electrictransducer unit according to an embodiment of the present invention.

FIG. 8 is a view showing wiring and the arrangement of electrodes on asecond insulating substrate of a piezo-electric transducer unitaccording to this embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below with thedrawings. FIG. 7 is a perspective view of components of a piezo-electrictransducer unit according to this embodiment. The piezo-electrictransducer unit comprises an insulating substrate 1 of ceramicinsulating materials, a thickness-mode crystal oscillator 2 (hereinafterreferred to as "AT crystal oscillator 2"), and a ceramic cap 3. Theinsulating substrate 1 is a laminate of a first insulating substrate 1aand a second insulating substrate 1b. A junction surface of the firstinsulating substrate 1a and second insulating substrate 1b, an uppersurface of the first insulating substrate 1a, and a lower surface of thesecond insulating substrate 1b are parallel to one another. The firstinsulating substrate 1a is thinner than the second insulating substrate1b and has a rectangular hollow part 4 in the center. The hollow part 4forms a concavity together with an upper surface of the secondinsulating substrate 1b when the first insulating substrate 1a is placedupon the second insulating substrate 1b to form the insulatingsubstrate 1. Also, the first insulating substrate 1a has separatinggrooves 5 and 6 for separating conductive adhesives. The separatinggrooves 5 and 6 are on opposite sides of a centerline of the insulatingsubstrate 1 parallel to its shorter sides.

The AT crystal oscillator 2 is of the so-called convex type and hasconnected to it an excitation electrode 21 and outgoing electrodes 22and 23, which are mounted fast on its upper and lower surfaces by vacuumevaporation, sputtering, or the like. The AT crystal oscillator 2 may bethe so-called rectangular AT plate of uniform thickness. After the firstinsulating substrate 1a and second insulating substrate 1b are bondedtogether, part of the AT crystal oscillator 2 is housed in the concavityformed on the insulating substrate 1 by the hollow part 4 and theoutgoing electrodes 22 and 23 are fixed, with conductive adhesives, ontoconnecting electrodes 7 and 10 or 8 and 9 (described below)respectively. As a result, the AT crystal oscillator 2 is mounted andfixed on the insulating substrate 1. The cap 3 is mounted on theinsulating substrate 1 like covering the AT crystal oscillator 2. So thecap 3, together with the insulating substrate 1, seal the AT crystaloscillator 2.

In FIG. 7, the upper surface (hereinafter referred to as "the mountingsurface") of the first insulating substrate 1a is where the AT crystaloscillator 2 is mounted, and is nearly square. For convenience of thefollowing explanation, it is assumed that a centerline of the nearsquare parallel to its longer sides is an X-axis and that a centerlineof the near square parallel to its shorter sides is a Y-axis. The hollowpart 4 is also nearly square and its centerlines are identical to theabove X- and Y-axis respectively. There are connecting electrodes atpositions contiguous to four corners of the hollow part 4. An X-Y planeformed by the above X- and Y-axis has four quadrants, each of which hasone connecting electrode. That is, the first quadrant has a firstconnecting electrode 7; the second quadrant has a second connectingelectrode 8; the third quadrant has a third connecting electrode 9; andthe fourth quadrant has a fourth connecting electrode 10. These fourconnecting electrodes 7, 8, 9, and 10 have perforating holes 7a, 8a, 9a,and 10a, respectively. These perforating holes perforate through thefirst insulating substrate 1a.

The separating groove 5 lies both on the first quadrant and on thefourth quadrant, and is arranged between the first connecting electrode7 and fourth connecting electrode 10 for preventing these electrodesfrom short-circuiting. The separating groove 6 lies both on the secondquadrant and on the third quadrant, and is arranged between the secondconnecting electrode 8 and third connecting electrode 9 for preventingthese electrodes from short-circuiting.

In FIG. 7, there are metallic wires on the upper surface of the secondinsulating substrate 1b; and a first external electrode 11, secondexternal electrode 12, third external electrode 13, and fourth externalelectrode 14 on its lower surface the locations of which correspond tothe first, second, third, and fourth quadrants on the above X-Y planerespectively.

FIG. 8 shows a pattern of the metallic wires on the upper surface of thesecond insulating substrate 1b, shown in FIG. 7, in detail. FIG. 8 showsthat there are perforating hole joints 7b, 8b, 9b, and 10b on the secondinsulating substrate 1b and that, when the first insulating substrate 1ais placed upon the second insulating substrate 1b, their positionscorrespond to the above perforating hole 7a, 8a, 9a, and 10a,respectively.

The first perforating hole joint 7b and second perforating hole joint 8bare connected by a first metallic wire 15. The first metallic wire 15 isarranged so that it bypasses the first external electrode 11 on thelower surface of the second insulating substrate 1b without crossing theX-axis. This prevents the shadows of the first metallic wire 15 andfirst external electrode 11 projected onto a plane parallel to the abovemounting surface from overlapping. The first metallic wire 15 isconnected to the second external electrode 12 by a second metallic wire16. The second metallic wire 16 reaches from the upper surface of thesecond insulating substrate 1b shown in FIG. 7 to the second externalelectrode 12 on its lower surface through a side corresponding to thelonger side of this surface. The second connecting electrode 8 isconnected to the third external electrode 13 by a third metallic wire 17through the perforating hole 8a and perforating hole joint 8b. The thirdmetallic wire 17 extends from the second quadrant having the secondconnecting electrode 8 to the third quadrant and reaches the thirdexternal electrode 13 through a side by bypassing the third perforatinghole joint 9b. Also, after the first insulating substrate 1a is placedupon the second insulating substrate 1b, the first and secondperforating hole joints 7b and 8b are connected, for example, withbrazing or an adhesive to the first and second perforating hole 7a and8a respectively. As described above, the first and second perforatinghole joints 7b and 8b and the second and third external electrodes 12and 13 are connected by the first, second, and third metallic wires 15,16, and 17. These connecting electrodes, external electrodes, metallicwires, perforating holes, and perforating hole joints form a first groupof the wiring pattern.

The third perforating hole joint 9b and fourth perforating hole joint10b are connected by a fourth metallic wire 18. The fourth metallic wire18 is arranged so that it bypasses the third external electrode 13 onthe lower surface of the second insulating substrate 1b without crossingthe X-axis. This prevents the shadows of the fourth metallic wire 18 andthird external electrode 13 projected onto a plane parallel to the abovemounting surface from overlapping. The fourth metallic wire 18 isconnected to the fourth external electrode 14 by a fifth metallic wire19. The fifth metallic wire 19 reaches from the upper surface of thesecond insulating substrate 1b shown in FIG. 7 to the fourth externalelectrode 14 on its lower surface through a side corresponding to thelonger side of this surface. The fourth connecting electrode 10 isconnected to the first external electrode 11 by a sixth metallic wire 20through the perforating hole 10a and perforating hole joint 10b. Thesixth metallic wire 20 extends from the fourth quadrant having thefourth connecting electrode 10 to the first quadrant and reaches thefirst external electrode 11 through a side by bypassing the firstconnecting electrode 7. As described above, the third and fourthperforating hole joints 9b and 10b and the fourth and first externalelectrodes 14 and 11 are connected by the fourth, fifth, and sixthmetallic wires 18, 19, and 20.

Further, after the first insulating substrate 1a is placed upon thesecond insulating substrate 1b, the third and fourth perforating holejoints 9b and 10b are connected, for example, with brazing or anadhesive to the third and fourth perforating hole 9a and 10arespectively. These connecting electrodes, external electrodes, metallicwires, perforating holes, and perforating hole joints form a secondgroup of the wiring pattern.

The above first and second group of the wiring pattern have differentpolarities when voltage is applied to the AT crystal oscillator 2. Also,these two groups of the wiring pattern are symmetrical in shape withrespect to the intersection of the X- and Y-axis, or the origin of theX-Y plane. Therefore, characteristics of the AT crystal oscillator 2 donot depend on whether it is connected to the first connecting electrode7 and fourth connecting electrode 10 or to the second connectingelectrode 8 and third connecting electrode 9.

According to the above embodiment, metallic wires are arranged on thebottom surface of the concavity formed on the insulating substrate 1 bythe hollow part 4, which gives a lot of space. As a result, space d₁between metallic wires belonging to the different groups can be widenedand length m₁ of a segment where a space between metallic wiresbelonging to the different groups is d₁ can also be shortened.Therefore, stray capacitance between metallic wires belonging to thedifferent groups becomes smaller. In this embodiment, the first metallicwire 15 and fourth metallic wire 18 are parallelly arranged in a nearlycentral part of the insulating substrate 1, where a segment having alength of m₁ has a space of d₁.

The reason for arranging metallic wires on the bottom surface of theconcavity formed on the insulating substrate 1 is that the insulatingsubstrate 1 is a laminate of the first insulating substrate 1a andsecond insulating substrate 1b where metallic wires etc. areindividually arranged.

Further, metallic wires and external electrodes are arranged so thatwhen they are projected onto a plane parallel to the above mountingsurface of the substrate, their shadows do not overlap. Therefore, straycapacitance between the metallic wires and the external electrodesbecomes smaller.

This results in smaller stray capacitance Cop between wires in theequivalent circuit shown in FIG. 5, and so smaller parallel capacitanceCo in the equivalent circuit shown in FIG. 6.

The first metallic wire 15 and fourth metallic wire 18 are arranged sothat they do not cross the X-axis, which prevents bad insulation even ifthe conductive adhesive for fixing the AT crystal oscillator 2 spillsonto the bottom surface of the concavity formed on the substrate.

A manufacturing experiment was conducted with a sample in which thethickness of the second insulating substrate 1b was 0.5-1 mm and thewidth 23 was 4-6 mm. The results showed that when space d₁ betweenmetallic wires corresponded to 1/3-1/10 of width w of the secondinsulating substrate 1b, capacitance Cop between the wires correspondedto 1/3 of that of a conventional example. When space d₁ between metallicwires becomes smaller than 1/10 of width w of the second insulatingsubstrate 1b, capacitance between the wires increases. And when space d₁between metallic wires becomes greater than 1/3 of width w of the secondinsulating substrate 1b, capacitance between the wires increases becausethe shadows of the metallic wires and external electrodes of the reverseprojected onto a plane overlap. As a result, when space d₁ betweenmetallic wires corresponds to 1/3-1/10 of width w of the secondinsulating substrate 1b, capacitance Cop between the wires has a nearlyminimum value. The thinner the second insulating substrate 1b becomes,the stronger this trend grows. Experimental results are shown in Table1.

                  TABLE 1                                                         ______________________________________                                        Equivalent                                                                      circuit constant  Conventional example  Present invention                   ______________________________________                                        Co (pF)     5.0           4.0                                                 ______________________________________                                    

Table 1 shows that a piezo-electric transducer according to the presentinvention has a smaller Co.

In the above embodiment, a concavity is formed on the insulatingsubstrate for housing the AT crystal oscillator, but an almost planeinsulating substrate may be used. In this case, wires are arranged on anAT crystal oscillator mounting area of an insulating substrate. Thisbrings about the same effect as the above embodiment. In this case, anAT crystal oscillator which is a rectangular AT plate of uniformthickness is more desirable than a convex type.

While what has been described is, at present, considered to be the bestembodiment of the invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaims cover all such modifications falling within the true spirit andscope of the invention.

We claim:
 1. A piezo-electric transducer unit comprising:apiezo-electric transducer having a first exciting electrode on a firstsurface and a second exciting electrode on a second surface; a substratehaving a mounting surface for mounting said piezo-electric transducer,said substrate having at least one first connecting electrode on a firsthalf of said mounting surface for connection to said first excitingelectrode of said piezo-electric transducer, at least one secondconnecting electrode on a second half of said mounting surface forconnection to said second exciting electrode of said piezo-electrictransducer, at least one first external electrode on said second half ofsaid substrate and at least one second external electrode on said firsthalf of said substrate, said first external electrode being on a secondsurface of said substrate, said second external electrode being on saidsecond surface of said substrate, a first wire for connecting said firstconnecting electrode and said first external electrode, and a secondwire for connecting said second connecting electrode and said secondexternal electrode; and a cap for covering the mounting surface and forsealing, together with said substrate, said piezo-electric transducerwith said piezo-electric transducer mounted on said substrate, wherein,when said first and second external electrodes and said first and secondwires are projected in a first direction onto a projection planeparallel to the mounting surface, the projected shadows of the firstexternal electrode and the second wire do not overlap and the projectedshadows of the second external electrode and the first wire do notoverlap, the first direction being perpendicular to said projectionplane.
 2. A piezo-electric transducer unit according to claim 1, whereinsaid substrate has a concavity in a nearly central position of themounting surface for housing at least part of said piezo-electrictransducer.
 3. A piezo-electric transducer unit according to claim 2,wherein said substrate is divided into a first substrate and secondsubstrate by a plane parallel to the mounting surface, said firstsubstrate has a perforated aperture corresponding to said concavity,said first and second connecting electrodes, and holes perforatingthrough said first substrate at said first and second connectingelectrodes, said second substrate has said first and second externalelectrodes, said first and second wires, and perforating hole jointsformed at positions corresponding to the perforating holes of said firstsubstrate, and part of said first and second wires are arranged at aposition corresponding to the bottom surface of said concavity.
 4. Apiezo-electric transducer unit according to claim 3, wherein theexternal shape of said substrate and said concavity are nearly square,two of said first and two of said second connecting electrodes arearranged on the four corners of said concavity so that they aresymmetrical with respect to a centerline parallel to the shorter sidesof said concavity, two of said first and two of said second externalelectrodes are arranged on the two longer sides of said substrate withone first and second external electrode per side so that they aresymmetrical with respect to a centerline parallel to the longer sides ofsaid substrate.
 5. A piezo-electric transducer unit according to claim3, wherein the total surface area of said first wire, first externalelectrode, and first connecting electrode is equal to that of saidsecond wire, second external electrode, and second connecting electrode.6. A piezo-electric transducer unit according to claim 5, wherein thesurface area of said first wire is equal to that of said second wire. 7.A piezo-electric transducer unit comprising:a piezoelectric transducerhaving a first exciting electrode on a first surface and a secondexciting electrode on a second surface; a substrate having a mountingsurface for mounting said piezo-electric transducer, a concavity in anearly central position of the mounting surface for housing at leastpart of said piezo-electric transducer, at least one first connectingelectrode for connection to said first exciting electrode of saidpiezo-electric transducer, at least one second connecting electrode forconnection to said second exciting electrode of said piezo-electrictransducer, at least one first external electrode and at least onesecond external electrode on a second surface of said substrate, a firstwire for connecting said first connecting electrode and said firstexternal electrode, and a second wire for connecting said secondconnecting electrode and said second external electrode; and a cap forcovering the mounting surface of said substrate and sealing, togetherwith said substrate, said piezo-electric transducer with saidpiezo-electric transducer mounted on said substrate, wherein saidsubstrate is divided into a first substrate and second substrate by aplane parallel to the mounting surface, said first substrate has aperforated aperture corresponding to said concavity, said first andsecond connecting electrodes, and holes perforating through said firstsubstrate at said first and second connecting electrodes, said secondsubstrate has said first and second external electrodes, said first andsecond wires, and perforating hole joints formed at positionscorresponding to the perforating holes of said first substrate, and atleast part of said first and second wires are arranged on an area of thesecond substrate corresponding to the bottom surface of said concavity.8. A piezo-electric transducer unit according to claim 7, wherein theexternal shape of said substrate and said concavity are nearly square,two of said first and two of said second connecting electrodes arearranged on the four corners of said nearly square concavity so thatthey are symmetrical with respect to a centerline parallel to theshorter sides of the near square, two of said first and two of saidsecond external electrodes are arranged on the two longer sides of saidnearly square substrate with one first and second external electrode perside so that they are symmetrical with respect to a centerline parallelto the longer sides of the near square.
 9. A piezo-electric transducerunit according to claim 8, wherein said first and second wires do notcross within said concavity a centerline parallel to the longer sidesthereof.